1. Field of the Invention
The present invention relates to a planetary gear type speed change device for use in an automatic transmission for a vehicle such as an automobile.
2. Description of the Prior Art
Planetary gear type speed change devices adapted for use in an automatic transmission for a vehicle and operable to provide five or six forward speed stages have been proposed in the forms of a combination of a double pinion type simple planetary gear mechanism and a Ravingeau type planetary gear mechanism (Japanese Patent Publication 48-27863 (1972)), a combination of three single pinion type simple planetary gear mechanisms (Japanese Patent Publication 50-32913 (1975)) and a combination of a double pinion type simple planetary gear mechanism and a single pinion type simple planteary gear mechanism (Japanese Patent Laying-open Publication 61-117950 (1986)).
When the speed change device can provide a larger number of forward speed stages, the difference between the rotational speed of the pump and the turbine of the fluid torque converter during the speed stage shifting becomes smaller, and thereby the power transmission efficiency of the torque converter is correspondingly increased, and the fuel economy of the vehicle is correspondingly improved. Further, if the rotational speed difference between the pump and the turbine of the fluid torque converter during the speed stage shifting becomes smaller, the shock which would occur during the speed stage shifting will also be reduced.
From the above view point it is desirable that the speed change device can provide a larger number of forward speed stages without increasing the overall speed change ratio between the lowest forward speed stage and the highest forward speed stage.
However, the speed change device generally becomes more complicated in the structure requiring more clutches and/or brakes when it is designed to provide a larger number of speed stages, resulting in an increase of the weight and the cost of the speed change device.